The embodiments described herein relate generally to an ion mobility spectrometer (IMS) and, more particularly, to an IMS detection system for enhancing detection of materials of interest through enhanced resolution of high-mobility ions and low-mobility ions.
At least some known spectroscopic detection devices include a time-of-flight (TOF) ion mobility spectrometer (IMS) detection system. Such TOF-IMS detection systems are used to detect trace portions of materials of interest, e.g., residues, in the presence of interfering substances in collected samples. In at least some known TOF-IMS detection systems, ions are generated in an ionization region to increase the ion population therein. An ion gate (sometimes referred to as an ion shutter) that includes a conducting grid of interleaved wires, e.g., a Bradbury-Nielson gate, is maintained in a “cut-off” condition that is configured to prevent an ion current to transmit from the ionization region. Energizing the ion gate deflects the ions in the ionization region to the gate wires, thereby collecting the ions and preventing them from flowing through the gate. De-energizing the ion gate allows the ions to flow out from the ionization region into a drift region, where a time-of-flight spectrum is generated. Based on an ions' mass, charge, size, and shape (the ion mobility), the migration time through the drift region is characteristic of different ions, leading to the ability to distinguish different analyte species.
The population of ions generated in the ionization region include low-mobility analytes and high-mobility analytes. The low-mobility analytes traverse the drift region with a lower velocity than the high-mobility analytes due to their relatively larger molecular size than the high-mobility analytes. The low-mobility and high-mobility analytes pulsed into the drift region from the ionization region typically generate spectrum peaks on spectral analysis equipment.
Both the sensitivity associated with the amplitude of the peaks (for low-level detection) and resolving power associated with the width of the peaks (to distinguish between close spectrum peaks) of the instrument are important. The ion gating time is the temporal period that the ion gate is de-energized. The resolving power for any particular analyte increases as the gating time is reduced because the initial peak width decreases. The increased resolving power and the associated narrower peak widths facilitate improved detection resolution. Notably, the sensitivity decreases rapidly below a certain ion gating time, thereby defining a lower limit for the ion gating time for an ion species of given mobility.
Ions of lower mobility and longer drift times have wider spectrum peak widths as a function of diffusion in the axial direction along the drift region as compared to higher mobility ions with shorter drift times. Therefore, for the same gating injection time and same initial ion current in the source region, the peak heights for the lower mobility ion have smaller values as compared to higher mobility ions. The lower mobility ions will exhibit higher spectrum peaks, i.e., increased sensitivity with a longer gating pulse. The higher mobility ions will also have higher spectrum peaks with longer gating pulses, but a significantly lower resolving power will result, thereby decreasing the resolution of the spectrum peaks. Therefore, the optimum ion gating time is different for every analyte mobility peak and the optimum ion gating time should be proportional to the drift time of a particular ion. As such, since known general purpose TOF-IMS systems need to sample ions of many species, often simultaneously, the sensitivity and the resolving power for a range of analytes during the period of a sample, usually 3-10 seconds, are not optimized for each of the analytes. Specifically, resolution of spectral traces of high-mobility analytes is sacrificed for retaining sensitivity for low-mobility analytes.